Radiation images represented by X-ray images are employed in a number of fields, such as for medical diagnoses. Mainly utilized as a method for preparing the X-ray images is a so-called radiographic system in which radiation, which has passed through an object, is irradiated onto a phosphor layer (also called a fluorescent screen) resulting in visible light which exposes a silver halide light-sensitive photographic material (hereinafter referred to simply as a light-sensitive material) and the resulting light-sensitive material is subjected to photographic processing to prepare a visible image.
In recent years, however, a new method has been proposed in which images are directly captured from a phosphor layer instead of the image forming method employing light-sensitive materials comprising silver halides.
The aforesaid method comprises a process of rendering a phosphor to absorb the radiation which has passed through an object, subsequently a process of exciting the resulting phosphor employing light or heat energy so that radiation energy, which has been stored by said phosphor through absorption of X-rays, is emitted as fluorescence, and a process of forming images while inspecting the resulting fluorescence. Specifically, the method refers to a radiation image conversion method utilizing a stimulable phosphor described, for example, in U.S. Pat. No. 3,859,527 and Japanese Patent Publication Open to Public Inspection No. 55-12144.
This method utilizes a radiation image conversion panel comprising stimulable phosphors. In more detail, radiation, which has passed through an object, is incident to the stimulable phosphor layer of the radiation image conversion panel and radiation energy corresponding to transmitted radiation intensity of each portion of the object is stored. Thereafter, the resulting stimulable phosphor is sequentially subjected to stimulation, employing electromagnetic waves (stimulation light), such as visible light, and infrared rays, so that radiation energy stored in the stimulable phosphor is released as stimulated luminescence. The resulting signals, depending on variation of light intensity, are subjected, for example, to photoelectric conversion to obtain electrical signals. The resulting signals are employed to reproduce visible images on conventional image recording materials such as silver halide light-sensitive photographic materials or on image display apparatuses such as a CRT.
Compared to radiography in which conventional radiographic light-sensitive materials and intensifying screens are employed in combination, the aforesaid reproduction method of radiation image recording exhibits advantages such that it is possible to obtain radiation images with ample information, while utilizing substantially reduced radiation exposure.
These radiation image conversion panels comprise a support having thereon a stimulable phosphor layer or a self-supporting stimulable phosphor layer. The stimulable phosphor layer is comprised of stimulable phosphors as well as binders which disperse-support the stimulable phosphors, or is comprised of only coagulated phosphors formed by a vacuum evaporation method or a sintering method. Further, also known are those in which voids in the coagulated phosphor are impregnated with polymers. Further, generally provided on the surface opposite the support side of the stimulable phosphor layer is a protective layer such as a polymer film or an inorganic material vacuum-evaporated layer.
As noted above, these stimulable phosphors are phosphors which result in stimulated luminescence when stimulation light is irradiated after irradiating radiation. In practice, phosphors are commonly used which result in stimulated luminescence in the wavelength region of 300 to 500 nm, utilizing stimulation light in the wavelength region of 400 to 900 nm. Stimulable phosphors which have conventionally been employed in the radiation image conversion panel include, for example, rare earth element activated alkaline earth metal fluorinated halide based phosphors described in Japanese Patent Application Open to Public Inspection Nos. 55-12145, 55-160078, 56-74175, 56-116777, 57-23673, 57-23675, 58-206678, 59-27289, 59-27980, 59-56479, and 59-56480; divalent europium activated alkaline earth metal halide based phosphors described in Japanese Patent Application Open to Public Inspection Nos. 59-75200, 60-84381, 60-106752, 60-166379, 60-221483, 60-228592, 60-228593, 61-23679, 61-120882, 61-120883, 61-120885, 61-235486, and 61-235487; rare earth element activated oxyhalide phosphors described in Japanese Patent Application Open to Public Inspection No. 59-12144; cerium activated trivalent metal oxyhalide phosphors described in Japanese Patent Application Open to Public Inspection No. 58-69281; bismuth activated alkaline metal halide based phosphors described in Japanese Patent Application Open to Public Inspection No. 60-70484; divalent europium activated alkaline earth metal halophosphate phosphors described in Japanese Patent Application Open to Public Inspection Nos. 60-141783 and 60-157100; divalent europium activated alkaline earth metal borate phosphors described in Japanese Patent Application Open to Public Inspection No. 60-157099; divalent europium activated alkaline earth metal hydrogenated halide phosphors described in Japanese Patent Application Open to Public Inspection No. 60-217354; cerium activated rare earth element composite halide phosphors described in Japanese Patent Application Open to Public Inspection Nos. 61-21173 and 61-21182; cerium activated rare earth element halophosphate phosphors described in Japanese Patent Application Open to Public Inspection No. 61-40390; divalent europium activated cerium-rubidium halide phosphors described in Japanese Patent Application Open to Public Inspection No. 60-78151; divalent europium activated halogen phosphors described in Japanese Patent Application Open to Public Inspection No. 60-78153; and tetradecahedron rare earth metal activated alkaline earth metal fluorinated halide based phosphors deposited from a liquid phase, described in Japanese Patent Application Open to Public Inspection No. 7-233369.
Of the aforesaid stimulable phosphors, iodine-containing divalent europium activated alkaline earth metal fluorinated halide based phosphors, iodine-containing divalent europium activated alkaline earth metal halide based phosphors, iodine-containing rare earth element activated rare earth oxyhalide based phosphors, and iodine-containing bismuth activated alkaline metal halide based phosphors result in stimulated luminescence with high luminance.
Radiation image conversion panels, which employ these stimulable phosphors, store radiation image information and subsequently release stored energy after being scanned with stimulation light. As a result, after such scanning, it is possible to repeatedly store radiation images. One advantage of such a radiation image conversion panel is its repeated usability. Namely, in conventional radiography, radiographic materials are consumed for every image capture. Contrary to this, in the radiation image conversion method, it is more advantageous from the viewpoint of resource conservation as well as economic efficiency, because it is possible to repeatedly use the same radiation image conversion panel.
Under the aforesaid practice, it has been strongly desired that a radiation image conversion panel is durable for use over an extended period of time without deteriorated image quality of the resulting radiation images.
However, when the aforesaid stimulable phosphors, which are employed to produce the radiation image conversion panel, are set idle under common weather conditions over an extended period of time, problems occur in which characteristics are degraded during storage due to humidity as well as radiation such as ambient ultraviolet rays.
For example, when a stimulable phosphor is set aside under highly humid conditions, radiation sensitivity decreases due to an increase in adsorbed moisture. Further, when set aside in a place which is exposed to high energy radiation such as ultraviolet radiation, the stimulable phosphor is subjected to partial decomposition, resulting in a decrease in radiation sensitivity. Generally, the latent image of a radiation image fades over time after irradiation of radiation. As a result, the strength of reproduced radiation image signals characteristically decreases as the time from irradiance of radiation to scanning by stimulation light increases. Further, as stimulable phosphor absorbs moisture, the rate of the aforesaid latent image fading accelerates, resulting in critical problems. When stimulable phosphor, which has absorbed moisture or has degraded due to radiation such as ultraviolet radiation during reading of the resulting radiation image, reproduction of subsequent signals is degraded.
In order to minimize the aforesaid degradation of stimulable phosphors due to moisture absorption, methods heretofore have been proposed and employed in which a stimulable phosphor layer is covered with a moisture resistant protective layer which exhibits low moisture permeability, or phosphor particles are subjected to a hydrophobic treatment so that less moisture reaches the phosphor layer.
Known as methods for preparing low moisture permeable protective layers are those in which glass plates or thick high-barrier resinous films are employed, or laminated films are employed which are prepared by laminating 2 to 8 layers of film which comprises polyethylene terephthalate film vacuum-evaporated with a thin glass layer comprised of metal oxides and silicon nitrides. However, when the aforesaid methods are employed, problems occur in which the sharpness of the resulting radiation images is degraded due to an increase in thickness of the protective film itself.
Further, the use of an aluminum oxide (alumina) evaporated layer results in excellent moisture resistance. However, especially when the alumina evaporated layer is prepared so as to have high moisture resistance, problems have still occurred in which light intensity, emitted by stimulable phosphors, decreases due to absorption of the emitted light by the alumina-evaporated layer.
Japanese Patent Application Open to Public Inspection No. 11-249343 describes an embodiment in which the entire phosphor plate is protected by employing a sealing material. However, when alkali halide phosphors exhibiting high moisture absorbability are employed, the resulting moisture resistance is insufficient. As a result, further improvements are desired.